I've kept a diary once in my life, when I was 15. At 16, having realised it was a bomb waiting to go off, I buried it in a dustbin. I haven't ever regretted it. I wonder if my aunt felt the same. In a move far more decisive than mine, she torched her diaries in a bonfire. The third diarist in my family is my great-grandfather, who fought in the first world war. The many letters he wrote to my great-grandmother aren't exactly a diary, yet they document his experiences in a similar way. They are now in the Imperial War Museum, a thick binder of pages written during the gore and filth of total war. He didn't do anything as extreme as my aunt or me – we edited our pasts into ash and landfill – but my great-grandfather's letters are still edited. The words are brave and considered rather than raw and desperate, since they were for his wife, who must have been sick with worry and had three young sons to care for. I'm sure he never imagined me, the granddaughter of one of those boys, reading his letters 100 years later.

I am equally sure Captain Scott never intended for anyone else to read his diaries, although I have done (for my new novel, Everland), as have countless other people, and there is a chance my great-grandfather did too. Ernest Shackleton, however, would not have been surprised: he edited his 1914-17 journal into the book, South!, which was published three years after he had returned from Antarctica. Scott's journal, in contrast, was retrieved from his pocket after he had been dead for eight months. The difference would prove to be important.

With curious symmetry, both Scott and Shackleton's lives ended up being defined by a journey of around 800 miles. With his ship Endurance crushed by the ice and the crew eventually marooned on Elephant Island, Shackleton and five men then sailed more than 800 miles in a boat to South Georgia to get help. Incredibly, they made it. It took another four months before Shackleton was able to rescue the stranded men, but he succeeded. Not a single man died. South! describes one of the most astonishing journeys ever made. And despite the overwhelming probability that no one from Endurance would survive, a spirit of cheerfulness permeates the book. Shackleton and his boat crew battle against terrifying odds with unbreakable optimism, while on the storm-battered Elephant Island, where the men were reduced to boiling old seal bones for food, there are comic anecdotes and banjo concerts. Any mention of the conflict or anguish that occurred is brief to the point of non-existence. As Shackleton remarks in the preface, the story is of "high adventure, strenuous days, lonely nights, and, above all, records of unflinching determination". Writing retrospectively, his focus is naturally on the larger triumph of their escape rather than the smaller, spikier details of their ordeal.

The story of Scott's last 800 miles, however, is one of grinding torture. Beaten to the South Pole by Amundsen, he slowly returns and slowly dies. His account is drawn from the perspective of someone whose chances of survival are being steadily destroyed. It displays just as much "unflinching determination" as Shackleton's, yet despair seeps from the page as the distance that Scott and his four men have left to travel becomes impossible to square with diminishing supplies and deteriorating health.

"Loneliness is the penalty of leadership," Shackleton wrote, which is possibly why Scott unburdened himself so freely on paper. He had also confided in diaries since his teenage years. In his last entries Scott reveals a man who could be depressive, snappish and critical. People irritate him, their work dissatisfies him and he makes biting asides about incompetence. But he was equally impatient with himself. His rigid belief in self-discipline was the result of a lifelong disgust at his own inclination towards laziness. While in the navy, he wrote "of the hope of being more worthy; but how shall I ever be?" This relentlessness of Scott's was often a beneficial quality. "The immense shove of the man," is how Apsley Cherry-Garrard, present on that 1910‑13 Antarctic voyage, described Scott's capacity for endurance. But his willingness to punish himself was also, as Ranulph Fiennes writes in his book about Scott, comparable to "self‑flagellation".

"Scott's diary, had he lived, would have formed the basis of the book he would have written," Cherry-Garrard notes. There is no doubt Scott planned to revise his diary into something more selective. I'm certain Shackleton revised his diary, just as I'm certain anyone would. Such edits aren't necessarily a misrepresentation of events, but reflect the diplomacy of hindsight. As Captain Oates, who died two weeks before Scott, once wrote to his mother: "Please remember that when a man is having a hard time he says hard things about other people which he would regret afterwards." For my great-grandfather, and the hundreds of thousands of other men who were sent to fight on the western front a year later, Scott's story was an inspirational example of courage. It continued to be so for several decades, until a newer, determinedly sensational form of biography became fashionable and Scott's admissions of self-doubt were used to destroy his character and explain the failure of his expedition. He was flawed, as all people are, but the extremely low temperatures and awful conditions that thwarted him and his men weren't of his making.

Shackleton didn't initially receive the same recognition as Scott. It took another 30 years after his death in 1922 before he began to gain similar levels of public renown, just at the time that Scott started to decline in popularity. Both men rest in the places that forged their reputations. Scott is still on the Ross Ice Shelf, buried somewhere beneath a century of snow, and Shackleton is buried on South Georgia, after he suffered a fatal heart attack on another Antarctic voyage one month before his 48th birthday.

Their most famous expeditions are technically about failure. Scott failed to reach the pole first, and Shackleton failed to be the first to cross the Antarctic continent. Of course, to classify them only as failures is deliberately to misunderstand what makes these two expeditions so extraordinary. Shackleton and Scott tell their stories in their own words, yet Scott's contains the reflexive frustration and turmoil of a man writing privately at the end of each day. He never had the chance to appraise the situation objectively, so we read it as he experienced it, blind to what the next 24 hours will bring. It makes for a more intimate, but perhaps less comprehensive account. It is a revealing story, certainly, but not the whole story.